NKT cells are a unique innate lineage of lymphocytes which express a T cell receptors of limited diversity and recognize glycolipid antigens presented on MHC-like CD1d molecules. A subset of NKT cells express an invariant TCR1 chain, Va14-Ja18 in mice, and are thus named iNKT (invariant NKT cells). NKT cells respond quickly to antigenic stimulation, producing significant quantities of cytokines and chemokines within minutes or hours, which regulate the immune response. In addition to having an important role in the clearance of a subset of alpha-proteobacteria, iNKT cells have also been shown to regulate tumor surveillance and tumor rejection, autoimmune diseases, and graft versus host disease (GVHD). Because of their anti-tumor effects, clinical trials are underway to mobilize and activate iNKT cells in fighting cancer. iNKT cells develop via a distinct pathway from 'conventional' a T cells, although they share CD4+CD8+ (DP) thymocytes as a common precursor. We have recently cloned a novel transcriptional repressor, NKAP, from a genetic complementation screen. Previously, we demonstrated that NKAP is required early in T cell development, as conditional deletion of NKAP, using an Lck-cre transgene, results in an early block in a T cell development but leaves ?d T cell development unaffected. To understand the function of NKAP later in a T cell development, we generated CD4-cre NKAP cKO mice, which delete NKAP efficiently at the DP stage of thymocyte development. Loss of NKAP expression in DP T cells leads to a complete abrogation of iNKT cell development, while conventional a T cell development is unaffected. This defect in iNKT cell generation is not due to lack of CD1d expression in DP T cells, nor in their expression of two Slam family receptors that are critical to iNKT cell development, Slamf1 (also known as CD150) and Slamf6 (also known as Ly108). In this proposal, we will define the molecular cause(s) for the failure in the development of iNKT cells in the absence of NKAP.